Process for rapid reignition of in situ combustion



United States Patent "ice 3,398,793 PROCESS FOR RAPID REIGNITION OF INSITU COMBUSTION Harry W. Milton, Jr., Denver, Colo., assignor t0Marathon Oil Company, Findlay, Ohio, a corporation of Ohio No Drawing.Filed May 27, 1966, Ser. No. 553,317 8 Claims. (Cl. 166-39) ABSTRACT OFTHE DISCLOSURE The present invention comprises a method for reigniting acombustion zone in an organic-bearing formation comprising incombination the steps of injecting into a formation having a burned-outzone and an unburned organic-bearing zone, a quantity of a flammablegaseous mixture comprising oxygen and a fuel at least sufficient tosubstantially fill said burned-out zone, thereafter igniting saidflammable mixture through at least one well located in said burned-outzone and located at least 10 feet from the interface between saidburned-out zone and said organic-bearing zone, so as to produce a gascombustion front which passes through a portion of the burned-out zoneand reignites the carbonaceous material in place at the interface,between said burned-out zone and said organic-bearing zone.

The present invention relates to new methods for the reignition of thecombustion of organic material in underground formations, and isparticularly useful in connection with the recovery of oil by in situcombustion methods.

The well known in situ combustion process involves the ignition ofcarbonaceous material in a stratum with organic content around awellbore as a result of heating and injection of air. A combustion frontis established which moves radially out from the wellbore with continuedair injection. Recovery of hydrocarbons and combustion gas productstakes place through one or more other wells.

A difficulty with in situ combustion oil recovery operations is that thecombustion front may be extinguished, either because of inadvertentreductions in the air supply, in homogeneities in the formation, orother reasons. When the combustion front is so extinguished, reignitioncan be extremely difficult. Previous attempts at solving this problemhave included the flooding of the burned-out zone with flammablepetroleum fractions, a process requiring large amounts of such flammablefluids with considerable resultant expense and delay. Another method isthat of US. 2,793,697 which heats the zone adjacent to the injectionwell to a temperature of above 1000 F. then injects nonflammable gasesto move the heat from the hot zone outward from the wellbore, thenbegins to inject flammable gases and immediately burns them close to thewellbore and continues this process cyclically so as to force a hot zoneoutward from the wellbore until this zone reaches the interface betweenthe burned-out and the unburned-out portions of the formation at whichpoint the oil in place is reignited. As stated in US. 2,793,697, thisprocess requires many days to ignite an interface located within tenfeet of the wellbore.

In contrast, the present invention permits the reignition of interfaceswhere combustion died out at much greater distances from the wellbore,and accomplishes this reignition -very rapidly without the need for therepititious cyclic operation described above. Further, the presentinvention utilizes gaseous hydrocarbons which can be natural gas orother gaseous fuels readily available in most oil fields.

The present invention involves the injection of a flam 3,398,793Patented Aug. 27, 1968 mable mixture of oxygen-containing gas,preferably air, and fuel, preferably gaseous hydrocarbons, into theburned-out portion of the formation, preferably either through a well orwells originally used for ignition or an original producing well orWells. When an amount of the combustible gaseous mixture which is atleast suflicient to fill the burned'out area has been injected, themixture is conventionally ignited. Ignition of such a flammable gaseousmixture in the burned-out portion of a forma tion according to theinvention, causes a gas combustion front which can travel rapidlyoutward from the point of ignition toward the interface between theburned-out area and the nonburned out area. On reaching this interfacethe gas combustion front ignites the carbonaceous material in place andoil recovery may then be reinstituted according to the conventional insitu combustion techniques.

The preferred fuels for use with the present invention are the naturalgas commonly present in oil and gas fields, methane, ethane, propane,butane, acetylene, ethylene, propylene and butylene.

While the point of ignition of the hydrocarbon gases must be in theburned-out portion of the formation, the flammable gas mixtures can beinjected at any of a number of points. A convenient point for injectionwould be an existing air injection well if combustion cocurrent to thedirection of gas flow is desired. An existing production well normallyused for Withdrawal of production gases or recovered hydrocarbons wouldbe the point of injection if countercurrent combustion is to beutilized. However, a special well may be drilled for the purpose wheredesired. Where the organic-bearing formation is sufficiently permeableto the flow of gas, a gas mixture may be injected at a point within thenon-burned out portion of the formation.

The pressure under which the flammable gas mixture is injected willpreferably be that normally used for injection of air into theformation. In most cases, the approximate volume of the burned-out areawill be known and the quantity of gas necessary to fill the burnedoutarea and sustain the gas combustion front can be readily calculated.However, in some instances it may be necessary to try successivelyincreasing quantities of flammable gas mixture and increasing gas flowrates until the oil in place has been reignited. Because of differencesin the volumes, shapes, depths and reservoir characteristics involved inthe formations in which reignition is to be accomplished, the variousparameters necessary to cause movement of the combustion front accordingto the present invention will vary relatively widely. For example, theinjection pressure used for injecting the flammable gas mixture will bedependent on the permeability of the formation and also will be limitedby the strength of the overburden, i.e. the depth' of the well. As arough indication, the pressure will generally be from one to about 2,000atmospheres with pressures of from 10 to about 1,000 atmospheres beingmore preferred, and pres sures of from about 20 to about atmospheresbeing most preferred in the most commonly encountered types ofreservoirs.

Also, flow rates of the flammable gas mixture will vary greatlyaccording to the formation differences given above. Generally, fuelgas-air fluxes will be from about 10 to about 2,000, preferably from 100to about 1,000 and most preferably from 200 to about 800 standard cubicfeet per hour per square foot of reservoir formation. The exactcomposition of the flammable gaseous mixtures can also vary Widelyaccording to the results desired and the reservoir involved. In general,however, a stoichiometric mixture of oxygen and combustible gas will beinjected into the formation. Also for better control of the com- 3bustion front, it may be desired to use a catalyst in con junction withprocess as described in US. 3,127,935.

The reignition of the oil in place can be determined by analysis of thecombustion gases emitting from a production well.

As soon as reignition has been accomplished, normal air feed is resumedand hydrocarbon recovery proceeds as before the in situ combustion frontwas extinguished.

While the present invention may be employed to establish reignition atinterfaces which are very close to the well through which reignition isestablished, its advantages will be more apparent when such interfacesare located more than ten feet or even more than one hundred feet awayfrom the well used for reignition.

The following example will further illustrate the invention, but shouldnot be taken as limiting it. The process of the invention is adaptableto a wide variety of modifications and variations which will be obviousto a person skilled in the art, from the reading of this disclosure, andall such obvious modifications and variations are to be taken asincluded within the scope of the claims appended hereto.

EXAMPLE In situ combustion is established in a strata of oil bearingsandstone and continues with normal injection of the air and productionof hydrocarbons and combustion gases until the oil combustion frontforms a roughly 100 foot radius circle concentric with the air-injection'Well. At this time, due to an insufficiency in the air injected, thecombustion front is extinguished. Rapid reignition of the oil combustionfront is accomplished by injecting into former production wells amixture of approximately 9.5% by volume methane with 90.5% by volumeair. Injection is continued until the entire burned-out portion of thereservoir is filled with the flammable methane-air mixture. An electricigniter located at the bottom of the previous air injection well is thenactuated causing ignition of the methane-air mixture. A gas combustionfront radiates rapidly outward from the base of the air injection well,passing through the porous burned-out portion of the rock formation. Onreaching the interface between the burnedout and the non-burned-outportions of the formation, the high temperatures of the gas combustionfront reignites the carbonaceous material in place. When it is observedfrom the analysis of combustion products that the in situ oil combustionprocess has been reestablished, then the methane-air supply may be shutolf. Air injection is then continued as before, but perhaps at asomewhat greater flow rate and the production of hydro- 4 carbons andcombustion gases from the withdrawal wells resumes normally.

What is claimed is:

1. A method for reigniting a combustion zone in an organic-bearingformation comprising in combination the steps of injecting into aformation having a burned-out zone and an unburned organic-bearing zone,a quantity of a flammable gaseous mixture comprising oxygen and a fuelat least sufiicient to substantially fill said burned-out zone,thereafter igniting said flammable mixture through at least one welllocated in said burned-out zone and located at least 10 feet from theinterface between said burned-out zone and said organic-bearing zone, soas to produce a gas combustion front which passes through a portion ofthe burned-out zone and reignites the carbonaceous material in place atthe interface, between said burned-out zone and said organic-bearingzone.

2. The process of claim 1 wherein the gaseous mixture consistsessentially of a combustible mixture of air and hydrocarbons have from 1to around 4 carbon atoms; and wherein flammable mixture continues to befed into the burned-out zone after initial ignition of the flammablemixture.

3. The process of claim 2 wherein the hydrocarbons comprise a majorportion of methane.

4. The process of claim 2 wherein the hydrocarbons comprise a majorportion of ethane.

5. The process of claim 2 wherein the hydrocarbons comprise a majorportion of acetylene, ethylene or propylene.

6. The process of claim 2 wherein the flammable gaseous mixture isinjected through at least one well located in the burned-out portion ofthe formation.

7. The process of claim 2 wherein the flammable gaseous mixture isinjected through at least one Well located in the organic-bearingnon-burned-out portion of the formation.

8. The process of claim 1 wherein the organic-bearing formation isselected from the group consisting of tarsands, oil shale, and liquidpetroleum-bearing formation.

References Cited UNITED STATES PATENTS 2,793,697 5/1957 Simm et a1.166-39 2,901,043 8/1959 Campion et al. 166-11 3,055,422 9/1962Schleicher et a1. 166-11 3,127,935 4/1964 Poettmann et a1. 166-11STEPHEN I. NOVOSAD, Primary Examiner.

